The PI's long-term goal is to achieve a fuller, more complete understanding of parent-offspring relations in mammals, at both the proximate and ultimate level of causation. The specific and immediate objective is to integrate the study of proximate and ultimate causation through a comparative analysis of the variation and flexibility in the proximate mechanisms that mediate paternal behavior and that prevent males from harming young. The proximate mechanisms will vary between and within species, and within the individual over time depending upon the adaptive task for which they were designed. Male behavior towards infants varies between and within species and is generally correlated with variation in mating systems and social/sexual experiences; thus variation and flexibility in the proximate mechanisms mediating paternal behavior and the prevention of harm to young is expected to be correlated with mating systems and parental rearing strategies. The proposed comparative research is designed to examine, in the laboratory, the social/sexual experiences that mediate paternal behavior and the prevention of harm to young in three species of Peromyscus that vary in their mating systems (monogamy to polygyny) and paternal care patterns (from extensive male care to little male care). It will also investigate individual differences in flexibility of these proximate mechanisms by examining changes in proximate mechanisms within the individual as he moves from his normal mating system to another, and whether the individual's reliance on a particular mechanism under one mating system helps or hinders a facultative change to reliance on a different mechanism under another mating system.